A thin film transistor (TFT) in an array substrate may have an active layer made of an oxide semiconductor. The oxide semiconductor has a high mobility and can be used in large size products. However, the oxide semiconductor has some drawbacks such us unstable properties, which usually results in undesirable effects such as threshold voltage (Vth) shift; hence, its application is limited. One reason of the unstable properties of the oxide semiconductor is that, a lattice structure of the oxide semiconductor does not match that of a gate insulating layer in the TFT and lots of defects are generated at the interface therebetween, these defects may capture charges and then cause the undesirable effects.